A glow discharge lamp is an electro-mechanical structure that allows creation of a plasma or ionized gas. The plasma formed in the glow discharge lamp is commonly referred to as a “glow discharge.” A glow discharge has applications including the bulk and composition depth profile (CDP) analysis of various materials by using the plasma for sputtering of the sample material. For these types of applications, control of the glow discharge is significant to obtain meaningful results. Methods of creating a glow discharge include the application of direct current (DC) or radio frequency (RF) energy to the glow discharge lamp. When a sample material is nonconductive, RF energy should be used.
Three electrical parameters, voltage, current, and power are measurable in a DC glow discharge. To accurately control a glow discharge, two of the three electrical parameters should be managed or measured for proper interpretation of the analytical results. The third electrical parameter can be calculated from the quotient or product of the other two. The ratio of voltage to current in the glow discharge has a direct influence on sputter rate and light emission. Power can also be utilized to control or predict sputter rate and light emission, but accuracy is improved if either the voltage or current is also determined. Sputter rate control is important when performing CDP analysis of a multilayer material as one of the goals is measurement of the various layers' thicknesses. To accomplish control of the sputter rate, it is typical for the glow discharge lamp pressure to be altered to achieve the desired voltage to current ratio at a given power level. In a glow discharge, increasing lamp pressure relative to absolute vacuum results in a lowering of the plasma impedance or plasma voltage to current ratio as more gas molecules are present for ionization.
When the glow discharge is driven with alternating current (AC) or RF power, it is convenient to compare electrical parameters with their DC equivalent values. The term “effective” is used to describe an amount of AC or RF stimulus that produces the same effect as a DC stimulus of the same magnitude. For example, in an RF system, effective plasma current refers to the amount of plasma current that must be applied to obtain the same effect as that obtained with an equal amount of DC current. In a DC glow discharge, effective electrical values are equal to the static potentials or currents applied.
Quantitative or composition RF glow discharge has historically been problematic due to deficiencies in the ability to precisely measure RF electrical parameters of the discharge. Effective voltage is the voltage at the sample surface where the plasma is present. Effective lamp voltage is the voltage measured at the RF electrode point of measurement. For example, effective voltage is only measureable for conductive samples since a nonconductor forms a capacitive divider between the RF electrode point of measurement and the actual voltage potential present at the sample surface and experienced by the plasma. Effective lamp voltage is equal to effective voltage only when the sample is conductive. Total lamp currents in an RF glow discharge lamp are complex comprising of capacitive sinusoidal currents as well as discharge-related sinusoidal and non-sinusoidal currents. The capacitive currents are developed from the mechanical structure used to create or confine the discharge structure or lamp. The complex plasma current comprised of both ion and electron current was described by H. S. Kino and G. S. Butler in “Plasma Sheath Formation by Radio-Frequency Fields,” The Physics of Fluids, Vol. 6, No. 9, September 1963, pp. 1346-1355. Techniques for measuring the current of an RF glow discharge lamp for elemental analysis are detailed by Ludger Wilken, Volker Hoffmann, Peter Geisler, and Klaus Wetzig (2004, Nov. 23) in U.S. Pat. No. 6,822,229. Wilken applied techniques for current, power and impedance measurement developed by C. Beneking (1990, Nov. 1), in “Power dissipation in capacitively coupled rf discharges,” J. Appl. Phys, 68 (9), pp. 4461-4473 to an RF glow discharge lamp used for elemental analysis.
RF power measurements should account for system losses and variation in operational voltages. There are multiple methods for determining the effective plasma power including True Plasma Power™ (TPP) also known as effective power (EP) described by K. A. Marshall, T. J. Casper, K. R. Brushwyler, and J. C. Mitchell (2003) in “The analytical impact of power control in a radio frequency glow discharge optical emission plasma,” J. Anal. At. Spectrom 18, pp. 637-645 or a vector multiplication technique implemented by L. Wilken, V. Hoffmann, H. J. Uhlemann, H. Siegel, and K. Wetzig (26 Feb. 2003 on Web) in “Development of a Radio-Frequency Glow Discharge Source with Integrated Voltage and Current Probes,” JAAS. 
Although conductive sample sputter rate correlation between RF and DC conditions is possible, see K. A. Marshall, T. J. Casper, K. R. Brushwyler, and J. C. Mitchell (2003), “The analytical impact of power control in a radio frequency glow discharge optical emission plasma,” J. Anal. At. Spectrom 18, 637-645, the ability to directly determine the effective RF plasma voltage or current for nonconductive samples or thin nonconductive layers has not been realized. Attempts to calculate the effective voltage based on sample capacitance have been demonstrated, but this method requires knowledge of the dielectric types and thicknesses involved, see L. Wilken, V. Hoffmann, and K. Wetzig (2005, Jun. 11), “Radio frequency glow discharge source with integrated voltage and current probes used for sputtering rate and emission yield measurements at insulating samples,” Anal Bioanal Chem, pp. 424-433 and L. Wilken, V. Hoffmann, and K. Wetzig (2007), “Electrical measurements at radio frequency glow discharges for spectroscopy,” Spectrochimica Acta Part B, 1085-1122.
The methods developed previously can only determine effective plasma power for both conductive and nonconductive samples. Effective voltage can only be measured for conductive samples. Although effective current can be calculated from the quotient of effective power divided by effective voltage, a method for determination of effective current from total lamp current has yet to be realized. The inability to measure the effective voltage or current for all sample types limits the capability to fully control the plasma, thereby placing limitations on the types of samples that can be accurately analyzed in both bulk and CDP experiments.